skidesmond wrote:From an observational point of view (because I'm not an engineer) I see the bladder exerting 40psi in all directions because my pressure gauge tells me so. So whether it's in a press, sitting on a table or floating in the air, it's exerting 40 psi.
when this blader is resting on your table the only pressure it does exerts on it, is its weght divided by the contact area between them.
the air in the tube is exerting 40 psi to the tube's walls. the air pressure is stressing only but the tubes walls. you can't break the table by pressing more air in there, but you can blow the tube appart. (well not realy true, cause if that bladder is strong enough and your compressor even stronger, you may pump in there such a mass of air tha 'll break the table with its weight)
skidesmond wrote:Now assume we have the hose w/ 40psi in a press with a cat track measuring 72in x 12in. The cat track will have 34,560 lbs of force exerted on to it ( 72*12*40=34560) which will be directed to the ski.
I agree except for one detail that I think needs clarification. In your example the hose with 40psi is exerting 34560 lbf on the cat track. This will only be the case if the contact footprint of the inflated bladder is the exact same size as the cat track. In other words 864in^2 for a bladder 72" long by 12" wide (12"is the inflated contact surface width and not the layflat width of the hose).
Because pressure is a function of force and area, the 40 psi will only be applied to the surface area at which the pressure is applied. This means that the flatter, and therefore, wider you can get the hose when pressurized, the more force your bladder can exert. I hope that made sense.
Last edited by WhitePine on Mon Feb 13, 2012 11:48 am, edited 1 time in total.
After reading this I think that a press should be designed to accommodate "shims" so that you can pack up the bottom mould to get the desired gap between the 2 moulds. This allows you can achieve the perfect width from the hose at pressure.
gozaimaas wrote:After reading this I think that a press should be designed to accommodate "shims" so that you can pack up the bottom mould to get the desired gap between the 2 moulds. This allows you can achieve the perfect width from the hose at pressure.
yep, although it's typically easier to shim down your top mold, rather than lifting your bottom mold. This also gives your hose more room up above. My press is crazy cramped, so my hose comes out of the mold, and is almost immediately squeezed against the top of the press, not great.
I love this thread. I just want to clarify to people something I've been thinking about recently while reading the forum. A lot of time pressure is thrown around as a measure of comparing one person's press to another. Some press at 80psi ,some at 45psi, and others at 14psi. It is hard to compare without knowing the context of the press setup.
One person's 50 psi is not equal to another person's 50 psi unless they have the exact same pnuematics, cat track, molds, top to bottom mold gap, and ski shape and length. This is all due to the reasons Fa explained.
Assuming two people have the same equipment but one presses a ski and the other presses a snowboard, the snowboard will have less pressure applied than the ski because of the increased surface area. However the force exerted on the ski/snowboard would be the same.
Thanks guys you are a huge help to someone like me who is in the design stages. Every new press can be better than the ones before it with help like this.
This is the most Educational part of this whole forum! I am going to a Engineering and science school in Wausau. I really needed information like this. So if you guys could come to a conclusion than I would appreciate it. I'll also ask my science and math teacher about this.
Assuming two people have the same equipment but one presses a ski and the other presses a snowboard, the snowboard will have less pressure applied than the ski because of the increased surface area. However the force exerted on the ski/snowboard would be the same.
Regarding the shim. I cannot imagine a case where it is better to shim down the top mould than it is to shim up the bottom mould. Unless you do not fix your top mould to the press, let gravity work for you and shim up the bottom by simply lifting it up, placing the shim under it and letting the mould sit on top. To shim the top you must have do undo the fixing mechanism, perform the same procedure and then redo the fixing mechanism. Seems like more work for zero gain to me.
gozaimaas wrote:Seems like more work for zero gain to me.
The gain is having space for your hose to go.
Also, to shim the bottom mold you have to lift the bottom mold (heavy? awkward?). To shim the top, you just let it drop down, and put crap on top of it. Less work, lots of gain?
In the end it really doesn't matter, and all depends on how your press is built, and how much room you have, which varies for everyone. In my mind the biggest benefit of keeping the bottom mold as low as you can is to have room for the hose to go as it leaves the tips angled upward.
